Femtosecond fluorescence upconversion studies of barrierless bond twisting of auramine in solution

Abstract

Femtosecond fluorescence upconversion studies have been performed for auramine ͑a diphenylmethane dye͒, dissolved in ethanol, as a function of temperature. It is found that the ͑sub͒picosecond decay components in the fluorescence slow down as the temperature is lowered from 293 K to 173 K. From the observation of a residual fluorescence, with a viscosity-dependent lifetime of about 30 ps ͑or longer at higher viscosity͒, and transient absorption results it is concluded that the two-state sink function model ͓B. Bagchi, G. R. Fleming, and D. W. Oxtoby, J. Chem. Phys. 78, 7375 ͑1983͔͒ does not apply in the case of auramine. Comparison of the auramine fluorescence kinetics in ethanol and decanol shows that diffusional twisting and not solvation is the main cause for the ͑sub͒picosecond excited state relaxation. To explain the experimental results, adiabatic coupling between a locally excited emissive state ͑F͒ and a nonemissive excited state ͑D͒ is considered. Torsional diffusion motions of the phenyl groups in the auramine molecule are held responsible for the population relaxation along the adiabatic potential of the mixed state, S 1 ͑comprised of the F and D states͒. Simulation of the excited state dynamics is feasible assuming a barrierless-shaped potential energy for S 1 and applying the Smoluchowski diffusion equation. The temporal behavior of the auramine band emission was simulated for the temperature range 293 K ϾTϾ173 K, with the temperature, T, and the viscosity coefficient, , being the only variable parameters. The simulated temporal behavior of the emission in the investigated temperature range is compatible with that obtained experimentally. The rotational diffusion coefficient for the auramine phenyl groups as extracted from the simulations is found to follow the Einstein-Stokes relation. From the numerical calculations the effective radius of the twisting phenyl groups is determined as 1.0 Å which compares well with the actual value of 1.2 Å.

A three-state effective Hamiltonian for symmetric cationic diarylmethanes

“…[4][5][6]12 The fluorescence enhancement is attributed to the suppression of a competing double-bond isomerization reaction, the rate of which is influenced by the local viscosity. [13][14][15][16] Consistent with their long history of use, the low-energy electronic excitations of dyes such as in Scheme 1 were an early target of quantum mechanical modeling efforts. [17][18][19][20][21] A consistent theme is the notion of resonance between "canonical" valence-bond states with opposing charge localization and bond alternation (cf.…”

A two-state model of twisted intramolecular charge-transfer in monomethine dyes

“…20,51,52 It has been remarked that the fluorescence decay times of fluorescent protein chromophore models in different protonation states can vary by up to an order of magnitude. 53 This effect cannot be accounted for by a purely viscosity-dependent mechanism (as has been invoked in other systems 40,42,45 ), because titration should not significantly change the molecular volume nor the surface area. 22 A study of a wide range of different chemical derivatives indicated that several mechanisms may be active, including internal conversion by largeamplitude motion but also possibly hydrogen-bond-assisted decay and/or charge-transfer, depending upon the derivative and the solvent used.…”

“…This behavior is predicted by very simple and general theoretical considerations. 31,32 It has not received much attention in the discussions of spectroscopic experiments, wherein it is often assumed that the dynamics can be described by a single effective coordinate [40][41][42][43]45 In order to identify whether twist-dependent charge transfer behavior and pathway bifurcation effects are relevant to understanding the twisting dynamics of monomethines, it is necessary to formulate simple physical models that can describe these phenomena for a general case. This is the point of this paper.…”

“…As a demonstration of the capacity of the model, we parameterize it against a selection of protonation states of the green fluorescent protein chromophore that sample electronic structures both near and far from the cyanine limit. 48,49 We claim that the model invokes a level of specific chemical detail intermediate between those used to generally describe barrierless photoisomerization in monomethines 22,40,42,64 and the atomistically SCHEME 1.…”

“…35 The excited-state twisting process in monomethine systems has been invoked as an example of an environmentcontrolled process with no intrinsic barrier. 21,28,[39][40][41][42][43][44] The dynamics have been described with theoretical models invoking overdamped motion. [40][41][42][43][44][45] Although the coupling of chargetransfer behavior to twisting displacements has been predicted for some time, 26 the theoretical models usually do not explicitly describe this.…”